This invention relates to a thermocouple protecting tube which can withstand the temperature measuring operation within molten metal for many hours resulting in the continuous measuring of the temperature of molten metal with high precision.
With the increase of continuous casting facilities in steel making plants, the continuous measuring of the temperature of molten metal has become a matter of vital importance.
The purpose for the above continuous measuring, which may vary depending on each steel making plant, can be summarized in view of two aspects, namely, quality control and the lowering of production costs.
Regarding quality control, continuous measuring is effective in the production of metal having a uniform quality and in the prevention of segregation within the metal. Furthermore, continuous measuring facilitates the refining operation since the temperature of molten metal can be continuously measured.
Regarding the lowering of production costs, continous measuring enables the complete computarization of the control of the refining operation and also improves the drawing speed of the continuous casting operation.
Conventionally, for measuring the temperature of molten steel, ceramic-made thermocouple protecting tubes, especially alumina protecting tubes, have been predominantly used.
However, when the alumina-made protecting tubes are directly immersed into a heat-molten body from an atmosphere of ordinary temperature, almost all the tubes rupture due to thermal shock which is caused by the rapid heating thereof, regardless of the size or shape of the protecting tubes.
For preventing the above mentioned rupture of the tubes, the protecting tubes are preheated by burners or by placing them over the heat molten body for a predetermined period of time whereby the operation becomes very laborious and time consuming. Although silicon nitride, aluminum nitride and silica glass can be considered as materials which have strong thermal-shock resistance, the first two materials are nitrides so that the oxidation thereof proceeds in open air places such as ordinary steel refining plants or glass producing plants and therefore they are not suitable as material for such a tube (the oxidation, however, does not occur in a vacuum or reduction atmosphere) and in general they are expensive. As for the third material, silica glass, strongly resists thermal shock, but it reacts easily with the platinum element of the thermocouple at high temperatures so that a temperature measurement of high precision is difficult and furthermore, due to the nature of silica glass, it easily reacts with molten metal or molten slag within a vessel whereby it can not withstand long usage thereof.
Of course, if the thickness of the protecting tube is considerably thicker than the usual thickness, the tube can withstand temperature and the long usage thereof. However, excessive thickness of the protecting tube also causes errors in measuring the temperature.
As another method for preventing the above rupture of tubes, a refractory powder coating is formed around the outer periphery of a thermocouple protecting tube to prevent the protecting tube from thermal shock. However, the above refractory powder coating is not sufficient for the above purpose and in addition it could happen that the protecting tube might break off during the transportation thereof.
Accordingly, it an object of the present invention to provide thermocouple protecting tubes which overcome the above mentioned problems and enable the continuous temperature measurement of molten metal.
It is another object of the present invention to provide a thermocouple protecting tube which is constructed such that the ceramic tube which includes a thermocouple element is concentrically disposed within an outer protecting tube which can withstand thermal shock which occurs when the protecting tube is immersed into molten metal.
It is still another object of the present invention to provide a thermocouple protecting tube which is constructed such that a ceramic tube which has on the outer periphery thereof, a refactory powder coated layer is disposed within a metal tube concentrically.
It is a further object of the present invention to provide a thermocouple protecting tube which is constructed such that a silica glass layer or a silica glass tube, which is of high thermal shock resistance, is formed around a ceramic tube.
These and other objects of the invention will become more apparent in the detailed description and examples as follow: